1. Field of the Invention
The present invention relates to a method of correcting a magnetization vector, and more particularly to a method of determining the vector of an unknown magnetization from the vector of a known magnetization when a mobile unit incorporating a stand-alone navigation system is magnetized by an external magnetism.
2. Description of the Prior Art
When an automobile passes through a railway crossing, a bridge, or the like, the magnetized state of body of the automobile tends to be varied, or the automobile body tends to be magnetized.
When the automobile body is magnetized, a geomagnetic sensor of the navigation system is also magnetically changed such that, as shown in FIG. 9 of the accompanying drawings, a magnetic circle shifts from C.sub.1 to C.sub.2 or the vector of the magnetization shifts from M.sub.1 to M.sub.2. The stand-alone navigation system is required to determine a direction vector from a new magnetization vector at all times. Therefore, if a direction vector were determined from the magnetization vector M.sub.1 before the magnetization of geomagnetic sensor is changed, then the direction vector would become R.sub.1 while the correct direction vector is in fact indicated by R.sub.2. As a result, the determined direction would be in error. A detected geomagnetic vector from a reference point 0 is represented by S.sub.A in FIG. 9.
To avoid the above error, there is known a turn correction or one-rotation correction process in which the automobile body is turned around 360.degree. to determine the new magnetization vector. The one-rotation correction process will be described below with reference to FIG. 10 of the accompanying drawings. As shown in FIG. 10, if maximum and minimum values V.sub.xmax, V.sub.xmin of a magnetization V.sub.x in an X direction, and maximum and minimum values V.sub.ymax, V.sub.ymin of a magnetization V.sub.y in a Y direction are known, then the coordinates of the center P of a magnetic circle, i.e., the coordinates of the tip end of a magnetization vector M, can be determined as follows: ##EQU1## Therefore, the magnetization vector M can be determined. This one-rotation correction process needs a space in which to rotate the automobile, and also requires a tedious and time-consuming procedure to be carried out for determining the magnetization vector M. Therefore, the one-rotation correction process has been putting a burden on the user of the navigation system.
Another known correction process which is free from the shortcomings of the one-rotation correction process employs an angular velocity sensor combined with a geomagnetic sensor. This correction process will be described below with reference to FIGS. 11A and 11B of the accompanying drawings.
First, an output signal from the angular velocity sensor is integrated, and thereafter converted into angle outputs v.sub.x, v.sub.y which are then plotted on the plane of geomagnetic sensor as shown in FIG. 11B.
The plotted point has coordinates (v.sub.x, v.sub.y). The difference or deviation between the output signals from the geomagnetic sensor and the angular velocity sensor is monitored at all times. The coordinates (V.sub.x, V.sub.y) of output signal from geomagnetic sensor is shown in FIG. 11A. The deviations .sigma..sub.x, .sigma..sub.y in X, Y directions between the output signals from the geomagnetic sensor and the angular velocity sensor are given as follows: EQU .sigma..sub.x =V.sub.x -v.sub.x (2), EQU .sigma..sub.y =V.sub.y -v.sub.y (3).
If no magnetic disturbance is applied to the geomagnetic sensor, then .sigma..sub.x =k.sub.1 (constant) and .sigma..sub.y =k.sub.2 (constant). These deviations are initialized, i.e., set to .sigma..sub.x =0 and .sigma..sub.y =0, and thereafter monitored. Then, when .sigma..sub.x, .sigma..sub.y &lt;k (k: a constant representing a threshold), it is determined that the automobile body is not magnetized, and when .sigma..sub.x, .sigma..sub.y .gtoreq.k, it is determined that the automobile body is magnetized.
As shown in FIG. 12 of the accompanying drawings, a deviation vector .SIGMA. (whose components are .sigma..sub.x, .sigma..sub.y) is added to a magnetization vector M.sub.n-1 prior to the magnetization of the automobile body, thus finding a new magnetization vector M.sub.n, as follows: EQU M.sub.n =M.sub.n-1 +.SIGMA. (4)
The above correction process which employs the angular velocity sensor can determine a new magnetization vector without imposing an undue burden on the user.
One problem with this correction process resides in the selection of a value for the threshold k. While no automobile body magnetization is assumed to occur when the values of .sigma..sub.x, .sigma..sub.y are less than the threshold k, a magnetic body is generally magnetized to a certain extent by a small magnetic field as it exhibits hysteresis unless subjected to a very small magnetizing force. Consequently, the correction process is not effective in correcting very small changes in magnetization vectors.